The proposed research of Project III concerns physiological aspects of cochlear implant function. The overall goal is to better understand the dependence of electrically evoked neural activity and electrode geometry, spatial patterns of electrical current, and temporal parameters of stimulus. This knowledge will guide the development of processing algorithms and the design and placement of electrode arrays in advanced applications of cochlear prostheses. The relationship between current fields and activation of neural tissue will be estimated in both temporal and spatial terms through measurements of voltage gradients induced in the implanted cochlea with multicontact arrays and of evoked field potentials and single-unit discharges in the manipulations, induced cochlear pathology, and computer simulations. The electrical stimuli used to define these mechanisms will include pulsatile and continuous waveforms (sinusoids and monosyllabic speech tokens), and models will be developed at the membrane, multiple-electronic-compartment, and multiple fiber levels. The validity of these models will be evaluated by comparing their predictions with single- and multiple-unit recordings. Many of these studies will have collaborative components in the biophysical and behavioral projects of this Program Project; physiological measures will provide verification of electroanatomical and biophysical models, and behavioral observations will be compared to predictions based on the physiological work.